1. Field of the Invention
The present invention relates in general to sidewall inspection devices for containers and in particular to a method and apparatus for comparing individual video data signals generated from an inspection of a container, such as a glass bottle.
2. Description of the Prior Art
The use of optical scanning devices for inspecting the sidewalls of containers is well known. Numerous devices, such as those shown in U.S. Pat. Nos. 3,708,680 and 3,716,136, have circuitry including means for receiving and interpreting light passed through or directed onto an item under inspection. Such devices incorporate either a visual display for comparison of the item or employ a device capable of producing a resistance proportional to the intensity of light directed thereon. Whether the output of such a device is visual or electrical in nature, it is eventually compared against a model to determine if the item under inspection is suitable as to size and construction and is without flaws, cracks, or foreign objects. Such devices are each intended to provide an automated inspection means for checking single or multiple objects, as in a moving column of bottles. U.S. Pat. No. 3,877,821 discloses an apparatus having a scanning array that is serially interrogated to generate a train of pulses having amplitudes representing the light transmitted through an object under inspection. Adjacent pulses are compared to generate pulses having amplitudes which represent the difference in the adjacent pulse amplitudes. The difference pulses can be utilized to indicate a defect in the object being inspected. U.S. Pat. No. 3,942,001 discloses an apparatus for detecting the presence of extraneous matter or cracks in translucent containers. A beam of light is projected through the container to generate an inspection signal which is compared with an acceptance signal. The acceptance signal amplitude is varied in accordance with the position of the spot beam with respect to the container.
One of the problems associated with prior art inspection devices is the sensitivity of the inspection device to general light variations across the container. For example, in the above discussed U.S. Pat. No. 3,877,821, the amplitude of the difference pulse varies in accordance with the intensity of the light. Thus, if the intensity of light varies across the container, a difference pulse representing one type of defect in one portion of the container may be different in amplitude than a difference pulse representing a similar defect in another portion of the container subject to a different intensity of light.